Radar level gauge (RLG) systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a waveguide. The probe is generally arranged to extend vertically from the top towards the bottom of the tank. The probe may also be arranged in a measurement tube, a so-called chamber, that is connected to the outer wall of the tank and is in fluid connection with the inside of the tank.
The transmitted electromagnetic signals are reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.
More particularly, the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and reception of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time (the so-called time-of-flight) and the propagation velocity of the electromagnetic signals.
Most radar level gauge systems on the market today are either so-called pulsed radar level gauge systems that determine the distance to the surface of the product contained in the tank based on the difference in time between transmission of a pulse and reception of its reflection at the surface of the product, or systems that determine the distance to the surface based on the phase difference between a transmitted frequency-modulated signal and its reflection at the surface. The latter type of systems are generally referred to as being of the FMCW (Frequency Modulated Continuous Wave) type.
For pulsed radar level gauge systems, time expansion techniques are generally used to resolve the time-of-flight.
Such pulsed radar level gauge systems typically have a first oscillator for generating a transmission signal formed by pulses for transmission towards the surface of the product contained in the tank with a transmitted pulse repetition frequency ft, and a second oscillator for generating a reference signal formed by reference pulses with a reference pulse repetition frequency fr that differs from the transmitted pulse repetition frequency by a given frequency difference Δf. This frequency difference Δf is typically in the range of Hz or tens of Hz.
At the beginning of a measurement sweep, the transmission signal and the reference signal are synchronized to have the same phase. Due to the frequency difference Δf, the phase difference between the transmission signal and the reference signal will gradually increase during the measurement sweep.
During the measurement sweep, the reflection signal formed by the reflection of the transmission signal at the surface of the product contained in the tank is being correlated with the reference signal, to form a measurement signal based on a time correlation between the reflection signal and the reference signal. Based on the measurement signal, the filling level can be determined.
Due to varying DC-offset levels and temperature drift etc. of electronic components comprised in the radar level gauge system some conditioning of the measurement signal is generally required for the radar level gauge system to be able to provide for an accurate filling level determination.
In some existing systems, the signal resulting from the above-mentioned time correlation is high pass filtered to make the measurement signal independent of varying DC-offset levels, temperature drift etc.
Since the relevant bandwidth of the measurement signal includes frequencies close to DC, a high pass filter with a very low frequency cut-off is used.
Such a high pass filter has a very slow step response, which means that the time from powering the radar level gauge system until measurements can start is rather long, in the range of seconds.
Accordingly, currently available pulsed radar level gauge systems typically need to be powered for a substantial period of time before the actual filling level measurement can start, which limits the capability of energy efficient intermittent operation of such radar level gauge systems.